Use a more sensible network topology for some router tests

This is achieved by creating subnets on separate logical
networks. A negative tests is also added to verify that
interface attachment fails when done with overlapping
subnets.

Closes-bug: #1316742

Change-Id: Idb1965e32d30acd962e1a3fa755b6c8b25c1bc31
Merge "Use a more sensible network topology for some router tests"